The present invention relates to semiconductor encapsulating epoxy resin compositions which provide cured products having outstanding fire retardance and free of the toxic substance antimony trioxide. The invention also relates to semiconductor devices encapsulated with these compositions in a cured state.
The semiconductor devices in use today are predominantly resin encapsulated diodes, transistors, integrated circuit (IC) chips, large scale integration (LSI) chips, and very large scale integration (VLSI) chips. Resin encapsulation is generally carried out with epoxy resin compositions because epoxy resins offer superior properties, (e.g., moldability, adhesion, electrical characteristics, mechanical characteristics, moisture resistance) compared with other thermosetting resins. Since semiconductor devices are used in all areas of our daily lives, including household appliances and computers, semiconductor encapsulants are required to be fire-retarding in the event that a fire occurs.
Halogenated epoxy resins and antimony trioxide (Sb2O3) are customarily included in epoxy resin compositions to increase the fire retardance. This combination of a halogenated epoxy resin with antimony trioxide has large radical-trapping and air-shielding effects in the vapor phase, thus conferring a high fire-retarding effect. However, halogenated epoxy resins generate noxious gases during combustion, and antimony trioxide has powder toxicity. Given their negative impact on human health and the environment, it would be preferable to entirely exclude these fire retardants from resin compositions.
Not only are resin compositions containing halogenated epoxy resins and antimony trioxide harmful to man and the environment, semiconductor devices encapsulated with these resin compositions have an inferior reliability when exposed to heat and moisture. This poor reliability arises because intermetallic compounds form at the junctions between aluminum electrodes and gold wire on the semiconductor device, causing an increase in electrical resistance and resulting also in wire breaks. The presence of the Brxe2x88x92 or Sb+ ions within the resin composition as part of the fire retardant is known to promote the formation of the intermetallic compounds.
In view of the above, studies have been conducted on the use of hydroxides such as Al(OH)3 and Mg(OH)2 or phosphorus-containing fire retardants in place of halogenated epoxy resins and antimony trioxide. Unfortunately, because of various problems associated with the use of these alternative compounds, such as inferior curability of the resin composition during molding and poor moisture resistance in the cured product, they are not yet ready for practical application.
It is therefore an object of the present invention to provide semiconductor encapsulating epoxy resin compositions which contain no halogenated epoxy resins or antimony trioxide, and yet have excellent fire retardance and reliability. Another object of the invention is to provide semiconductor devices encapsulated with these resin compositions in a cured state.
Accordingly, this invention provides semiconductor encapsulating epoxy resin compositions comprising (A) an epoxy resin, (B) a phenolic resin curing agent, (C) a fire retardant comprising zinc molybdate carried on spherical silica having a mean particle diameter of 0.2 to 20 xcexcm and a specific surface of 1 to 20 m2/g, and (D) an inorganic filler. These epoxy resin compositions provide cured products having a high fire retardance and excellent reliability, yet containing no halogenated epoxy resin or antimony trioxide.
The epoxy resin used as component (A) in this invention may be any epoxy resin having at least two epoxy groups per molecule, other than halogenated epoxy resins. Illustrative examples of suitable epoxy resins include novolac-type epoxy resins such as phenolic novolac epoxy resins and cresol novolac epoxy resins, triphenolalkane epoxy resins, aralkyl epoxy resins, biphenyl skeleton-containing aralkyl epoxy resins, biphenyl epoxy resins, heterocyclic epoxy resins, naphthalene ring-containing epoxy resins, bisphenol-type epoxy resins such as bisphenol A epoxy compounds and bisphenol F epoxy compounds, and stilbene epoxy resins. Any one or combination of two or more of these epoxy resins may be employed.
No particular limit is imposed on the phenolic resin serving as curing agent (B) in the invention, so long as the phenolic resin has at least two phenolic hydroxyl groups in a molecule. Illustrative examples of typical phenolic resin curing agents include novolac-type phenolic resins such as phenolic novolac resins and cresol novolac resins, naphthalene ring-containing phenolic resins, triphenolalkane resins, aralkyl phenolic resins, biphenyl skeleton-containing aralkyl phenolic resins, biphenyl phenolic resins, alicyclic phenolic resins, heterocyclic phenolic resins, naphthalene ring-containing phenolic resins, and bisphenol-type phenolic resins such as bisphenol A and bisphenol F. Any one or combination of two or more of these phenolic resins may be employed.
The relative proportions of the epoxy resin (A) and the phenolic resin curing agent (B) used in the epoxy resin compositions are not subject to any particular limits, although it is preferred that the amount of phenolic hydroxyl groups in the curing agent (B) be from 0.5 to 1.5 moles, and especially 0.8 to 1.2 moles, per mole of epoxy groups in the epoxy resin (A).
The semiconductor encapsulating epoxy resin compositions of the invention do not contain conventional fire retardants such as antimony trioxide and brominated or otherwise halogenated epoxy resins. Instead, the inventive compositions use as the fire retardant (C) a substance prepared by supporting zinc molybdate on spherical silica having a mean particle diameter of 0.2 to 20 xcexcm and a specific surface of 1 to 20 m2/g. Zinc molybdate by itself is known to have a smoke-reducing and charring effect in burning plastic, but it exists in the form of very fine particles and so cannot easily be dispersed in a resin composition. However, by supporting zinc molybdate on spherical silica having a mean particle diameter of 0.2 to 20 xcexcm and a specific surface of 1 to 20 m2/g, there is obtained a fire retardant which is well dispersible in resin compositions. This fire retardant does not cause any loss in flow or curability during molding, and makes it possible to obtain epoxy resin compositions having sufficient fire retardance and excellent reliability in the cured state without using a halogenated epoxy resin or antimony trioxide.
The shape, particle diameter, and distribution of the supporting filler (spherical silica) are crucial for achieving fire retardance using as little zinc molybdate as possible, and for maintaining or enhancing the moldability of the epoxy resin composition.
Therefore, the spherical silica used as the zinc molybdate carrier should have a mean particle diameter of 0.2 to 20 xcexcm, and preferably 0.3 to 10 xcexcm. One of several ways in which the mean particle diameter can be determined is as the weight average value (median diameter) using a particle size distribution measurement apparatus based on the laser light diffraction technique. Particles with a mean particle diameter smaller than 0.2 xcexcm are less dispersible within the resin compositions. A mean particle diameter greater than 20 xcexcm discourages uniform dispersion and support of the zinc molybdate, lowering the fire retardance. This in turn necessitates the use of a larger amount of the fire retardant, which is economically undesirable. The specific surface, as obtained by a suitable technique such as BET adsorption, is from 1 to 20 m2/g, and preferably from 2 to 18 m2/g. Particles with a specific surface of less than 1 m2/g retard the uniform support of zinc molybdate, resulting in a lower fire retardance. On the other hand, at a specific surface above 20 m2/g, dispersibility within the resin composition declines.
The zinc molybdate and the spherical silica serving as the carrier are used in relative proportions such that the content of zinc molybdate based on the total amount of fire retardant (i.e., the total amount of zinc molybdate and the spherical silica serving as the carrier) is preferably 1 to 50% by weight, and more preferably 5 to 40% by weight. A zinc molybdate content of less than 1% by weight would be difficult to achieve sufficient fire retardance, whereas a content greater than 50% by weight would make uniform support of the zinc molybdate on the spherical silica difficult to achieve.
The amount of fire retardant (i.e., the total amount of zinc molybdate and the spherical silica serving as the carrier) in the epoxy resin compositions of the invention is preferably 1 to 300 parts by weight, more preferably 3 to 200 parts by weight, and most preferably 5 to 100 parts by weight, per 100 parts by weight of the epoxy resin and the phenolic resin curing agent combined. Less than 1 part by weight of the fire retardant would fail to achieve a sufficient fire-retarding, whereas the use of more than 300 parts by weight would adversely affect the flow and curability of the composition during molding.
The zinc molybdate content within the fire retardant (i.e., the total amount of zinc molybdate and the spherical silica serving as the carrier) is preferably 0.02 to 35 parts by weight, more preferably 0.1 to 30 parts by weight, and most preferably 0.5 to 25 parts by weight, per 100 parts by weight of the epoxy resin and the phenolic resin curing agent combined. Less than 0.02 part by weight of zinc molybdate would fail to achieve a sufficient fire-retarding effect, whereas the inclusion of more than 35 parts by weight would lower the flow and curability of the composition during molding.
Within the fire retardant used herein, the uranium content is preferably not more than 10 ppb. At a uranium content higher than 10 ppb, the uranium content in the resulting epoxy resin composition becomes high enough for soft errors caused by xcex1 rays to arise in the semiconductor device when the composition is used as a memory chip encapsulant. To ensure that the fire retardant has a uranium content no higher than 10 ppb, a low xcex1 ray-grade spherical silica having a uranium content of less than 1 ppb is preferable for supporting the zinc molybdate.
The fire retardant used herein is an extremely effective fire retardant which has not been found to have a powder toxicity like that of antimony trioxide. Examples of commercial products that may be used as this fire retardant include Kemgard series such as Kemgard 1260, 1261, 1270, and 1271, all available from Sherwin-Williams Co.
The inorganic filler (D) included in the epoxy resin compositions of the invention may be any suitable inorganic filler other than the above-said component (C) commonly used in epoxy resin compositions. Illustrative examples include silicas such as fused silica and crystalline silica, alumina, silicon nitride, aluminum nitride, boron nitride, titanium oxide, and glass fibers. No particular limit is imposed on the mean particle diameter and shape of these inorganic fillers, although the use of spherical fused silica having a mean particle diameter of 5 to 40 xcexcm is preferred because it endows the epoxy resin composition with good molding and flow characteristics.
The inventive epoxy resin compositions have inorganic filler loadings of preferably 400 to 1,200 parts, more preferably 450 to 1,000 parts by weight per 100 parts by weight of the epoxy resin and the phenolic resin curing agent combined. At less than 400 parts by weight, the epoxy resin combination would have a large coefficient of expansion, resulting in greater stress on the semiconductor device and a decline in the device characteristics. Moreover, the proportion of resin relative to the overall composition becomes larger, sometimes failing to attain the fire retardance that is the object of this invention. On the other hand, more than 1,200 parts by weight of the inorganic filler would result in an excessive rise in viscosity during molding, and thus a poor moldability. The content of inorganic filler within the epoxy resin composition (excluding the above-described fire retardant (C)) is preferably 55 to 92% by weight, and especially 57 to 90% by weight.
The inorganic filler used herein is preferably surface treated beforehand with a coupling agent such as a silane coupling agent or a titanate coupling agent in order to increase the bonding strength between the resin and the inorganic filler. Preferred examples of such coupling agents include epoxy group-containing silanes such as xcex3-glycidoxypropyltrimethoxysilane, xcex3-glycidoxypropylmethyldiethoxysilane, and xcex2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; amino group-containing silanes such as N-xcex2-(aminoethyl)-xcex3-aminopropyltrimethoxysilane, xcex3-aminopropyltriethoxysilane, and N-phenyl-xcex3-aminopropyltrimethoxysilane; and mercaptosilanes such as xcex3-mercaptopropyltrimethoxysilane and xcex3-mercaptopropylmethyldimethoxysilane. No particular limitation is imposed on the amount of coupling agent used for surface treatment or the method of surface treatment.
In the practice of this invention, use is preferably made of a curing accelerator to promote the curing reaction between the epoxy resin and the curing agent. The curing accelerator may be any suitable substance that promotes the curing reaction. Illustrative, non-limiting examples of curing accelerators that may be used include phosphorus compounds such as triphenylphosphine, tributylphosphine, tri(p-methylphenyl)phosphine, tri(nonylphenyl)phosphine, triphenylphosphine triphenylborane, and tetraphenylphosphine tetraphenylborate; tertiary amine compounds such as triethylamine, benzyldimethylamine, xcex1-methylbenzyldimethylamine, and 1,8-diazabicyclo[5.4.0]undecene-7; and imidazole compounds such as 2-methylimidazole, 2-phenylimidazole, and 2-phenyl-4-methylimidazole.
The semiconductor encapsulating epoxy resin compositions of the invention may also include various additives, if necessary. Illustrative examples include stress-lowering additives such as thermoplastic resins, thermoplastic elastomers, synthetic organic rubbers, and silicones; waxes such as carnauba wax, higher fatty acids, and synthetic waxes; colorants such as carbon black; and halogen trapping agents.
The inventive epoxy resin compositions may be prepared by compounding the epoxy resin, curing agent, inorganic filler, and other components in predetermined proportions, thoroughly mixing these components together in a mixer or other appropriate apparatus, then melting and working the resulting mixture using hot rolls, a kneader, an extruder or the like. The worked mixture is then cooled and solidified, and subsequently milled to a suitable size so as to give a molding material.
The resulting epoxy resin compositions of the invention can be effectively used for encapsulating various types of semiconductor devices. The method of encapsulation most commonly used is low-pressure transfer molding. The epoxy resin composition of the invention is preferably molded at a temperature of about 150 to 180xc2x0 C. for a period of about 30 to 180 seconds, followed by postcuring at about 150 to 180xc2x0 C. for about 2 to 16 hours.
The semiconductor encapsulating epoxy resin compositions of the invention cure into products which have an excellent fire retardance. Owing to the absence of halogenated epoxy resins and antimony trioxide, the epoxy resin compositions have no adverse impact on human health or the environment. Moreover, these resin compositions have good flow and curing properties when molded, and provide excellent reliability in the cured state.